What is the correct measurement systems for animation?
The best way is using the reference unit. for each animation you should have a proper reference footage or etc.

For example you want to create walk cycle. you start to walk in your room and observe Walking variables by taking a footage from it. when you want to use that footage, you should use a measurement system With which you understand your footage. for this example metric unit maybe is the best choice.

But when you want to simulate a animation in big scale, you pick a proper ratio of a small unit that can describe the actual scale. for example the space between moon and earth is 363,104 kilometers. you can use metric unit and set it 3.63 meter to get the best result.